The invention relates to the general field of magnetic storage devices with particular reference to read heads in disk systems.
The principle governing the operation of most current magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve or SV. The resulting increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are a low coercivity (free) ferromagnetic layer, a non-magnetic spacer layer, and a high coercivity ferromagnetic layer. The latter is usually formed out of a soft ferromagnetic layer that is pinned magnetically by a nearby layer of antiferromagnetic material. Additionally, a synthetic antiferromagnet (formed by sandwiching an antiferromagnetic coupling layer between two antiparallel ferromagnetic layers) may be used to replace the ferromagnetic pinned layer. This results in an increase in the size of the pinning field so that a more stable pinned layer is obtained. We will refer to it as a synthetically pinned device.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance at this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to the film""s plane. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible while the resistance of the leads into and out of the device need not be particularly low. By contrast, in a CPP device, the resistivity of both the leads and the other GMR stack layers dominate and should be as low as possible.
As shown in FIG. 1, the CPP spin valve structure has three magnetic layers: free layer 17 as well as AP1 layer 15, and AP2 layer 13. Free layer 17 is free to rotate in response to external fields. The AP2 direction is fixed by antiferromagnetic layer 12 (typically MnPt) with ruthenium layer 14 being used to provide the antiferromagnetic coupling. Their relative magnetization directions of AP1 and AP2 during device operation are always antiparallel to one other. It is normal practice to utilize the same material (like CoFe) for both AP1 and AP2. This has a positive bulk spin asymmetry coefficient xcex2 as well as positive interface spin asymmetry coefficient xcex3.
xcex2 is defined as 1-xcfx81↑/(2xcfx81)=xcfx81↓/(2xcfx81)xe2x88x921 where xcfx81↑, xcfx81↓ are the resistivity of spin up and spin down electrons, respectively. xcfx81 is the material resistivity (=xcfx81↑xcfx81↓/xcfx81↑+xcfx81↓). xcex3 is defined as 1-r↑/2rb)=r↓/(r↑+r↓) where r↑(r↓) is the interface resistance for spin up and spin down electrons; rb=(r↑r↓)/r↑+r↓). When r↑=r↓, xcex3 will be 0 and the interface has no spin dependent scattering. Also seen in FIG. 1 is seed layer 11, capping layer 18 and non-magnetic spacer layer 16.
In TABLE I we show the xcex2 and xcex3 magnitudes for the three magnetic layers together with the resulting magnitude of their resistivity for both up and down electrons for both the parallel and antiparallel states:
The consequences of this are that the AP2 contribution to CPP GMR is always negative so it reduces the resistance contrast between the parallel and anti-parallel states of the free layer. This limits the GMR ratio as well as dRA (change between parallel and anti-parallel resistance) for synthetically pinned spin valves. This becomes even clearer when we compare a synthetic pin CPP SV with a single CPP SV (i.e. one lacking the AP1 and AP2 layers) by performing a two current channel model calculation. For this example, the calculation assumed the following two structures:
a. seed/MnPt200/CoFe30/Cu20/CoFe30/capxe2x80x94single CPP SV and
b. seed/MnPt200/CoFe20/Ru8/CoFe30/Cu20/CoFe30/capxe2x80x94Synthetic pin SV
The values computed for these two structures were found to be:
a. dRA=1.34 mohm/xcexcm2; RA=74.9 mohm/xcexcm2; GMR=1.7% and
b. dRA=0.66 mohm/xcexcm2; RA=78.5 mohm/xcexcm2; GMR=0.8%
This confirmed that both dRA and GMR were greatly reduced for the synthetically pinned CPP SV due to the AP2""s negative contribution. The present invention describes a structure, and process to form it, in which the contribution from AP2 is made to be positive, thereby enhancing both GMR and dRA greatly for a synthetic pin CPP SV.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,627,704, Lederman et al. show a MR CCP transducer structure. Dykes et al. (U.S. Pat. No. 5,668,688) shows a CPP SV MR device. U.S. Pat. No. 6,134,089 (Barr et al.) also describes a CPP MR device. U.S. Pat. No. 5,731,937 (Yuan) teaches a CPP GMR Transducer while Sakakima et al. disclose MTJ (magnetic tunnel junction) and CPP devices in U.S. Pat. No. 6,084,752. In U.S. Pat. No. 5,959,811, Richardson shows a CPP 4 terminal device.
It has been an object of at least one embodiment of the present invention to provide a Current Perpendicular to Plane Spin Valve (CPP SV) for use as a read head in a magnetic information storage system.
Another object of at least one embodiment of the present invention has been that the pinned layer of said CPP SV be synthetically pinned.
A further object of at least one embodiment of the present invention has been that said CPP SV have a performance that is at least as good as that of one having a directly pinned layer while continuing to enjoy the stability associated with a synthetically pinned layer.
Still another object of at least one embodiment of the present invention has been to provide a process for manufacturing said CPP SV.
These objects have been achieved by modifying the composition of AP2, the antiparallel layer that contacts the antiferromagnetic layer. Said modification comprises the addition of chromium or vanadium to AP2, while still retaining its ferromagnetic properties. Examples of alloys suitable for use in AP2 include FeCr, NiFeCr, NiCr, CoCr, CoFeCr, and CoFeV. The ruthenium layer normally used to effect antiferromagnetic coupling between AP1 and AP2 may be retained or may be replaced by a layer of chromium.